Currently, pi-conjugated polymers represent the most promising electron-donor materials for application in solution-processed organic bulk-heterojunction solar cell devices. Recently published studies (Nat. Photon. 2009, 3, (11), 649-653; J. Am. Chem. Soc. 2010, 132, (44), 15547-15549; Adv. Mater. 2010, 22, (46), 5240-5244) suggest that solar power conversion efficiencies on the order of those achieved with amorphous silicon technologies (ca. 10%) will be accessible upon identifying (i) proper molecular design and (ii) optimum device architecture. Nonetheless, a number of limitations encountered with conjugated polymers include: (i) batch-to-batch molecular weight and polydispersity variations, (ii) tedious material purifications, (iii) time-intensive syntheses, and (iv) solubility often limited to a small number of organic solvents. In contrast, conjugated small molecules are monodispersed, can be synthesized and purified with ease, and may ultimately be solution-processed from a variety of organic and aqueous solvents upon selecting proper solubilizing substituents.
Our work has been directed to introducing a design principle that would allow π-conjugated conjugated small molecules to arrange in an “end-to-end” fashion so as to mimic a polymeric backbone. In comparison with polymeric backbones which are composed of a multiplicity of covalently bound repeat units able to delocalize and transport charges with efficacy in a thin-film device, the degree of connectivity between conjugated small molecules is typically low and the charge transport in devices remains a critical limiting-parameter to date. Therefore, only a small number of reports (Adv. Funct. Mater. 2009, 19, (19), 3063-3069; Chem. Mater. 2010, 22, (7), 2325-2332; Appl. Phys. Lett. 2009, 94, (10), 103301; Adv. Mater. 2011, ASAP, DOI: 10.1002/adma.201004445) have described small molecules that yield power conversion efficiencies superior to 3% so far. The combination of (i) a lack of intermolecular connectivity and (ii) the formation of overly crystalline morphologies with domain-sizes exceeding the exciton diffusion length in thin-film devices, is known to be the bottle-neck of small molecule OPV device performance. In this regard, the identification of design principles that would allow for (i) enhancing the charge percolating pathway between molecules, (ii) tailoring the degree of microstructural order in small molecules in devices, and (iii) limiting the size of the crystalline domains formed across the active layer, is critically needed for this technology to compete with existing high-performing polymer solar cells strategies.
To the best of our knowledge, explicit strategies that promote intermolecular connectivity in order to mimic the material properties and photovoltaic device performance of a polymer backbone have not been described to date. Only strategies aimed at reducing the domain size of the small molecule crystallites formed during post-processing annealing treatments have been reported so far (e.g. Chem. Mater. 2009, 21, (9), 1775-1777). None of these reports describe a design principle that introduces the synthesis of conjugated small molecules which would arrange in an ordered “end-to-end” fashion, thus mimicking the backbone continuum of a conjugated polymer.